It has been found that the color intensity of cationic azo dyes, in particular of the specific azo dyes of formula (I), can be increased by use of specific anionic surfactants (b). A further subject of the present disclosure is therefore the use of anionic surfactants (b) for improving the color absorption capability of cationic azo dyes, in particular of the specific azo dyes of formula (I), on keratin fibers.
Generally, either direct dyes or oxidation dyes are used for the dyeing of keratin fibers. Intense colorings having good fastness properties can indeed be attained with oxidation dyes, however the color generally develops under the influence of oxidizing agents, such as H2O2, which in some cases can result in damage to the fibers. In addition, some oxidation dye precursors or specific mixtures of oxidation dye precursors can have a sensitizing effect in individuals who have sensitive skin. Direct dyes are applied under more gentle conditions. However, the disadvantage of these dyes lies in the fact that the colorings often have only insufficient fastness properties.
Depending on the desired color result, a person skilled in the art will use direct dyes of different dye classes. The direct dyes known from the prior art for example belong to the class of nitro dyes, anthraquinone dyes, azo dyes, triarylmethane dyes, or methine dyes. All of these dye classes should meet a specific requirement profile for use in the field of cosmetics. Direct dyes should thus deliver an intense color result and should have the best possible fastness properties. The color result obtained with direct dyes should be influenced as little as possible by ambient influences, i.e. the dyes should have a good fastness to washing, fastness to light, and fastness to rubbing, for example. Chemical influences to which the keratin fibers can be exposed after the coloring process (such as permanent waving) should change the color result to the smallest extent possible.
So as to also at the same time achieve a lightening with the dyeing, the direct dyes where possible should also be compatible with and stable in respect to the oxidizing agents (such as hydrogen peroxide and/or persulfates) usually used in the bleaching process.
Cationic azo dyes have proven to be a dye class having an excellent requirements profile. Azo dyes are generally characterized by a high stability. In addition, on account of their positive charge, cationic azo dyes have a high affinity to the keratin fiber, which is negatively charged to a greater or lesser extent.
If keratin fibers are to be oxidatively lightened or bleached, direct dyes can also be used in combination with oxidizing agents. Aqueous hydrogen peroxide solutions are generally applied to the keratin fibers for the bleaching of hair either alone or in combination with further oxidizing agents acting as bleach activators, such as persulfate salts. In order to attain a sufficient bleaching effect, agents of this type are usually set to be heavily alkaline, with the pH value here generally lying between 9 and 10.5. Melanin, i.e. the natural, color-giving pigments of the hair fibers, is oxidatively destroyed by the action of the oxidizing agents, and a decolorization or lightening of the fibers is achieved in this way. The melanin is localized in the cortex of the hair fibers and can be divided into two pigments classes. Eumelanins are the first, brown-black pigment class, whereas the reddish pigments richer in sulfur are referred to as pheomelanins. Due to the different resistances of the various pigment types with respect to oxidizing agents, however, the pheomelanins and eumelanins are not always decolorized uniformly. In addition, in darker hair having a high melanin content, the melanin can only be broken down in part or incompletely, such that a residual amount of the color-giving pigments remains in the hair after the bleaching. In these cases, the residual content of the melanin still present in the hair after the oxidative process leads to a yellowish to reddish nuance shift. When bleaching darker hair in particular, there is thus a color shift in the direction of warm (reddish) tones.
Such color shifts in the direction of warm tones are usually undesired by the user. This color shift is therefore usually counteracted by a tinting in the corresponding complementary color. Here, the objective is to achieve a silvery cooler impression of the bleach result. A person skilled in the art refers to a “matting” in this context.
For the matting of orangeish shades of blonde, blue direct dyes can be used in particular. For the most complete weakening possible of the orange color impression, it is advantageous here if the blue dye itself does not have any red component in its coloring. Dyes in pure blue tones are thus better, compared to purplish-blue dyes, for the matting of a bleaching result that is too orange.
Within the group of direct blue dyes that can be used in market products, there are only very few representatives which allow coloring in pure blue shades without red component. No dyes which meet all of the aforementioned preconditions in an optimal manner are known from the prior art. There is also a great demand for stable dyes which color the keratin fibers in pure blue shades and which deliver an intense color result with excellent fastness properties.
Monomeric cationic azo dyes that have long been known from the prior art are for example the representatives constituted by Basic Orange 31 (alternative name: 2-[(4-aminophenyl)azo]-1,3-dimethyl-1H-imidazolium chloride, CAS-Nr. 97404-02-9) and Basic Red 51 (alternative name: 2-[((4-dimethylamino)phenyl)azo]-1,3-dimethyl-1H-imidazolium chloride, CAS-Nr. 77061-58-6).
Both dyes color keratin fibers with excellent color intensity in the orange to red nuance range. There is additionally still also a need for direct blue dyes which are compatible in an optimal manner with these two dyes.